PURPOSE: To provide the polymer low toxic when used in vivo, excellent in biocompatibility and anti-thrombogenicity, thus useful for artificial blood vessels, etc., having specific chemical structure.

CONSTITUTION: The polymer having such structure that polymer segments of respective formulas I and II are virtually, alternately linked. In the formulas, n is 0-20; m is 0-10; R1 is a mixture of (A) 95-60mol% of at least one kind selected from (1) polyethers both terminated with alkylene groups with at least one kind of 3-8C alkylene linked mutually via ether linkages and (2) polyesters and/or polycarbonates both terminated with alkylene groups with at least one kind of 2-8C alkylene linked mutually via ester and/or carbonate linkages and (B) 5-40mol% of a polysiloxane both terminated with alkylene groups; R2 is divalent hydrocarbon ≤500 in molecular weight; R3 is such an amino acid that the second basic group is an amino group among basic amino acids and/or such a divalent hydrocarbon formed by eliminating two amino groups from a compound selected from biological amines bearing the two amino groups.